The best thing you can do is to learn from your mistake. Do absolutely everything you can to learn about every stage of labor, what is normal and what is not, what you can do and if you are unsure call the vet. Most are more than willing to share knowledge. Know your cows - keep notes from year to year on what their bags do before calving, what their tail set is like, even their attitude toward the rest of the herd and you. The more you know about what is "supposed" to happen and what is "normal" for your cows then the easier it will be to determine that something isn't right. And follow your gut...if your gut feelings are red flags then get the vet out!!
When you think you know everything - look around - I guarantee someone will have something you have never seen before.
There are lots of resources out there - just google "calving AND cow" and see what you get. Almost every Extension office will have something on calving, check out the closest University with an Ag program often they have short courses on managment or books/binders that you can order that detail common diseases, management, feeding, calving, weaning etc for your general area.
There is also a couple books by an English Vet, Eddie Straiton - Calving the Cow and Care of the Calf and Cattle Ailments: Recognition and Treatment (this covers diseases not calving) - both have LOTS of pictures (rather graphic). He uses a dead calf to demonstrate the dystocia positions and how to resolve them and when to get help.
Best of luck!